So what’s PUPSIT?
A readers query.
Introduction
Where terminal sterilisation is not possible – one of the more commonly used sterilisation methods is sterilising filtration of both gases and liquids.
Both gas and liquid filtration is based on the same fundamental principle of size exclusion filtration to remove microbes above 0.22 micron in size (a “primary control”) but the filtration of liquids bring unique technological challenges that result in the need for specialised secondary controls which verify the integrity of the filter and assure its performance.
What is PUPSIT?
PUSPIT stands for Pre-Use Post Sterilisation Integrity Testing.
Performing a PUPSIT allows, in combination with a post use integrity test, the highest possible confidence in the integrity of the sterilizing filter and all the surrounding or supporting elements such as the filter connections and housing.
In the context of post use integrity testing of filters (also mandatory), you might wonder why PUPSIT is needed but its guards against some specific risks.
1) False passes of post use testing. There is a possibility that a liquid with a high percentage of suspended solids (or bacteria) will blind a filter over time. When this occurs, there is a possibility that this blinding blocks a defect or tear in the filter. This leaves open a critical failure mode that a filter which should not pass the post use test will register a falsely passing result.
2) Damage post a pre use test. The requirement to evaluate post sterilisation and in situ guards against possible damage during the sterilisation process itself or damage incurred during installation. Damage which may not be detected due to defect masking as outlined above.
3) Business Risk. PUPSIT testing helps manage the risk of a filter failure to the business. While a masked filter causing a post use false pass is a theoretical possibility it’s not something that has been observed in practice in the publish literature beyond some very specifically designed experiments that where designed to demonstrate the phenomenon. However, filter test failure due to incorrect installation are fairly routine (if undesirable) and will often lead to (expensive) batch rejection.
Why do we have to do it?
The recently revised EU Annex 1 regulation came effective as of 25th August 2023 defines requirements for gases and liquid sterilisation by filtration including those shown below.
8.82 - The filtration system should be designed to:
vi. - Permit in-place integrity testing, of the 0.22 µm final sterilising grade filter, preferably as a closed system, both prior to, and following filtration as necessary. In-place integrity testing methods should be selected to avoid any adverse impact on the quality of the product.
8.87 - The integrity of the sterilised filter assembly should be verified by integrity testing before use (pre-use post sterilisation integrity test or PUPSIT), to check for damage and loss of integrity caused by the filter preparation prior to use. A sterilising grade filter that is used to sterilise a fluid should be subject to a non-destructive integrity test post-use prior to removal of the filter from its housing. The integrity test process should be validated and test results should correlate to the microbial retention capability of the filter established during validation. Examples of tests that are used include bubble point, diffusive flow, water intrusion or pressure hold test. It is recognized that PUPSIT may not always be possible after sterilisation due to process constraints (e.g. the filtration of very small volumes of solution). In these cases, an alternative approach may be taken providing that a thorough risk assessment has been performed and compliance is achieved by the implementation of appropriate controls to mitigate any risk of a non-integral filtration system. Points to consider in such a risk assessment should include but are not limited to:
In depth knowledge and control of the filter sterilisation process to ensure that the potential for damage to the filter is minimized.
In depth knowledge and control of the supply chain to include:
• Contract sterilisation facilities.
• Defined transport mechanisms.
• Packaging of the sterilised filter, to prevent damage to the filter during transportation and storage.
3. In depth process knowledge such as:
• The specific product type, including particle burden and whether there exists any risk of impact on filter integrity values, such as the potential to alter integrity-testing values and therefore prevent the detection of a non-integral filter during a post-use filter integrity test.
• Pre-filtration and processing steps, prior to the final sterilising grade filter, which would remove particle burden and clarify the product prior to the sterile filtration.
8.88 - The integrity of critical sterile gas and air vent filters (that are directly linked to the sterility of the product) should be verified by testing after use, with the filter remaining in the filter assembly or housing.
As with all sterile manufacturing the stakes here of not knowing what you are doing as a company are frighteningly high. Sterile filtration, if used, will be heavily integrated into you contamination control strategy and will be challenged as part of your media fill strategy. In terms of QP responsibilities, ensuring correct compliance to Annex 1 is a part of general GMP compliance for sterile medicines but ensuring both the filtration process itself and the integrity testing procedures are validated relate directly to ensuring point 1.7.12 is secure – (“…manufacturing and testing processes remain in the validated state.”)
Note: For products that are terminally sterilised but with a significant delay between manufacture and sterilisation the use of sterile filtration to eliminate bioburden and reduce the probability of product degradation by microbial action the use of the approaches below may also be desirable as part of a broader contamination control strategy (just wait until we get into that in depth).
Implementation challenges.
When performing a post sterilisation in situ, pre use test you need to maintain sterility on the downstream side of the filter membrane which will often require installing additional components for a filtration setup – more components, more failure points, more complexity in process.
As a reminder the integrity of the filter can be tested with two different approaches:
Forward flow test — This test measures flow rate through the porous structure at a defined differential pressure.
Bubble point test — This test determines the differential pressure required to expel the wetting liquid from the largest pores.
As integrity testing will use water to wet out the filter for testing, the requirement to perform post SIP can be challenging for process trains that require drying post sterilisation. The extra pipework needed to route testing liquid can also post changes for trains where minimisation of dead volume is a concern.
As a result PUPSIT tests can be complex, leaving more room to introduce errors that otherwise would not exist. Undetected errors could affect the efficacy and safety of the final product. The biggest risks are:
Manual processing errors (opening or closing the wrong valves at the wrong time) – generally favouring automated setups.
Leaks (from additional components like T-pieces and connectors)
Any errors in the more complex handling steps could introduce additional contaminants, negating any benefit of the PUPSIT process. This is why Annex 1 permits alternative risk managed alternatives where PUPSIT may not be possible after sterilisation due to process constraints. Your risk assessment needs to be robust and integrate into the wider contamination control strategy, for example the use of pre filtration to minimise the potential for filter blinding.
